Boeing and Bombardier traded verbal blows on Monday over claims by the US planemaker that the Canadian company receives massive subsidies.
The two sides appeared before the US International Trade Commission (ITC) in the latest round of their bitter row.
Boeing accused Bombardier of harming sales of its 737 aircraft and urged the ITC to support tariffs on its rival.
But Bombardier, which makes wings in Belfast, said Boeing makes "money hand over fist" from the 737.
Boeing claims Bombardier's new C-Series aircraft is being sold in the US below cost because of Canadian subsidies.
The US company won the first round of the fight in October when the US Commerce Department ordered that tariffs of up to 300% should be imposed on the C-Series.
The ITC will decide if the tariffs should be made permanent, which could effectively shut off the US market to the C-Series.
In opening remarks to the ITC hearing, Bombardier representative Peter Lichtenbaum said: "Boeing is making money hand over fist. And with a backlog of 737 orders years into the future, there are no signs of difficulty on the horizon."
Boeing countered that it had already been "established beyond question that Bombardier has taken billions of dollars in illegal government subsidies to prop up its C-Series programme. The C-Series would not even exist at this point but for those subsidies".
Canada's ambassador to the US, David MacNaughton, warned that a ruling in favour of Boeing would not be the end of the dispute.
He told the ITC panel that backing Boeing risked a possible violation of World Trade Organization rules.
"Boeing's assertion that future imports from Canada threaten to cause material injury is necessarily based on just the type of speculation and conjecture that is prohibited under both US and international law," he said.
Canada earlier this month scrapped plans to buy 18 Boeing Super Hornet fighter jets, underlining Ottawa's anger over the trade challenge.
The dispute is also being closely watched by the UK government, which fears any impact on C-Series sales will threaten jobs.
The dispute stems from a 2016 sale of 75 C-Series jets to Delta Air Lines. Boeing claims Delta paid $20m per plane, well below an estimated cost of $33m and what Bombardier charges in Canada.
Earlier this year, European planemaker Airbus took a controlling stake in the C-Series programme, and will begin production in Alabama. This will increase the US content of the aircraft, and generate hundreds of jobs.
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